Ventilation is the part of a kitchen that is invisible until it is wrong, and by then the duct is buried in a ceiling. It is also the part most often chosen on looks, which is how a beautiful 30 inch canopy ends up over a 36 inch pro range doing almost nothing.
The calculation for gas
Add up the BTU ratings of every burner on the cooktop. A common 30 inch gas range totals around 40,000 BTU; a 36 inch pro range is often 55,000 to 60,000.
Divide by 100. That gives you a CFM figure. Then round up to the next available model, because duct runs, elbows and filters all cost you airflow between the fan and the outside.
A 40,000 BTU range therefore wants around 400 CFM. A 60,000 BTU range wants 600 and realistically more if the duct run is long or has two elbows.
Electric and induction are not exempt
There is a widespread belief that induction does not need much ventilation because there is no combustion. There is no combustion product, true, but there is a great deal of steam and aerosolised fat, and induction gets a pan to temperature faster than gas does.
Plan 250 to 350 CFM for a 30 inch induction or electric cooktop. The one real saving is that you are not exhausting combustion by-products, so the make-up air threshold matters less often.
Duct size is not negotiable
- Up to 400 CFM: 6 inch round.
- 400 to 600 CFM: 7 or 8 inch round.
- 600 CFM and above: 8 or 10 inch round.
- Rectangular duct is fine if the equivalent area matches, but every transition costs airflow.
- Never reduce the duct below the hood's outlet size. A 600 CFM hood on a 6 inch duct is a 350 CFM hood that is loud.
The make-up air rule
Once a hood moves more than 400 CFM, it is extracting more air than a reasonably tight house can passively replace. The house goes to negative pressure, and the easiest path for replacement air becomes whatever flue is available, which may be a water heater or a fireplace.
Oregon, like most of the country, requires a make-up air path above that threshold. In practice that means a duct from outside with a motorised damper interlocked to the hood, so that when the fan runs the damper opens.
This has to be planned during framing. It is a duct, a damper, a control wire and often a small inline heater. Discovering it at inspection means opening a ceiling that has just been painted.
Mounting height and capture area
A hood works by capturing a rising plume before it spreads. Two things decide whether it does: how far above the cooktop it sits, and how much wider it is than the cooktop.
For a wall hood, mount 24 to 30 inches above the cooking surface and follow the appliance data plate, because clearance to combustibles is a code matter as well as a performance one. Gas usually wants the upper end of that range.
A hood should be at least as wide as the cooktop and ideally 3 inches wider on each side. Over an island, make it 6 inches wider on each side and add 100 to 200 CFM, because there is no wall helping to contain the plume and the room's own air currents work against you.
A short version
- Total BTU divided by 100, rounded up, for gas.
- 250 to 350 CFM for a 30 inch induction or electric cooktop.
- Match the duct to the hood, never smaller.
- Above 400 CFM, plan make-up air during framing.
- Wider than the cooktop, and wider again over an island.